Glass sheets and other substrates can be coated with a stack of transparent, metal-containing films to vary the properties of the coated substrates. Particularly desirable are coatings characterized by their ability to transmit visible light while reducing the transmittance of other wavelengths of radiation, especially radiation in the infrared spectrum. These characteristics are useful for minimizing radiative heat transfer while controlling visible transmission. Coated glass of this nature is useful as architectural glass and as automotive glass.
Low-emissivity coatings and other controlled transmission coatings typically include one or more infrared-reflective films and two or more antireflective transparent dielectric films. The infrared-reflective films reduce the transmission of radiant heat through the coating. The infrared-reflective films commonly are conductive metals (e.g., silver, gold, or copper), although transparent conductive oxides (e.g., ITO) or conductive nitrides (e.g., TiN) may also be used. The transparent dielectric films are used primarily to reduce visible reflection, to provide mechanical and chemical protection for the sensitive infrared-reflective films, and to control other optical coating properties, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium, as well as nitrides and oxynitrides of silicon. Such coatings can be deposited on glass sheets through the use of well-known magnetron sputtering techniques.
It is known to deposit a thin metallic layer directly over an infrared-reflective silver film to protect the silver film during deposition of a subsequent dielectric layer and/or during tempering or any other heat treatment. These protective layers (sometimes called “sacrificial layers” or “blocker layers”) have been formed of various materials, such as titanium, niobium, niobium-titanium, or NiCr.
The particular material from which the blocker layer is formed impacts various properties and characteristics of the coating. Titanium blocker layers, for example, have been found to impart excellent scratch resistance in low-emissivity coatings. They also adhere well to both an underlying silver film and an overlying oxide film. Niobium has been found to be an advantageous blocker layer material as well. In addition, niobium-titanium has been found to be particularly beneficial in certain respects.
It is sometimes necessary to heat coated glass sheets to temperatures at or near the softening point of glass (726 degrees C.), e.g., to temper the glass or enable it to be bent into desired shapes. Tempering is important for glass used in automobile windows, and particularly for glass used in automobile windshields, as well as in various architectural glazing applications. Upon breaking, tempered glass exhibits a break pattern in which the glass shatters into many small pieces, rather than into large dangerous shards. During tempering, coated glass is typically subjected to elevated temperatures on the order of about 700 degrees C. Moreover, the coated glass must be able to withstand such temperatures for substantial periods of time. Certain film stacks having silver as the infrared-reflective film are not able to withstand such high temperature processing without unacceptable deterioration of the silver film.
To avoid this problem, glass sheets can be heated (e.g., bent or tempered) before they are coated. The desired films can then be applied after heating. This procedure, however, tends to be complicated and costly and, more problematically, may produce non-uniform coatings.
In many cases, it is desirable for temperable low-emissivity coatings to have only an upper blocker layer (i.e., without any lower blocker layer). In other cases, a reflective silver film is protected from deterioration at high temperatures by sandwiching the silver between two metallic blocker layers. In such cases, the two blocker layers are thick enough and reactive enough that when the coated glass is heated to high temperatures, these films capture oxygen and/or nitrogen that would otherwise reach and react with the silver.
In addition to the infrared reflection provided by low-emissivity coatings, these coatings can provide desired shading properties. As is well known, the solar heat gain coefficient (SHGC) of a window is the fraction of incident solar radiation that is admitted through a window. There are a number of applications where low solar heat gain windows are of particular benefit. In warm climates, it is especially desirable to have low solar heat gain windows. For example, solar heat gain coefficients of about 0.4 and below are generally recommended for buildings in the southern United States. Further, windows that are exposed to a lot of undesirable sun benefit from having a low solar heat gain coefficient. For example, windows on the east or west side of a building tend to get a lot of sun in the morning and afternoon. For applications like these, the solar heat gain coefficient of a window can play a vital role in maintaining a comfortable environment within the building. Thus, it is beneficial to provide windows of this nature with coatings that establish a low solar heat gain coefficient (i.e., high shading ability coatings).
Tradeoffs are sometimes made in low-emissivity coatings in order to obtain the desired shading properties. For example, the films selected to achieve a low SHGC may have the effect of restricting the visible reflectance to a higher level than is ideal. As a consequence, windows bearing these coatings may have a somewhat mirror-like appearance. It would be desirable to provide a high shading ability coating that has sufficiently low visible reflectance to obviate this mirror-like appearance problem.
In addition to having undesirably high visible reflectance, the reflected or transmitted colors of conventional high shading ability coatings may be less than ideal. For example, these coatings may exhibit hues that are more red or yellow than is desired. To the extent a coating has a colored appearance, it is pleasing if the coating exhibits a transmitted and reflected hue that is blue or blue-green. The chroma of these coatings may also to be greater than is desired. In most cases, it is preferable to provide a coating that is as color neutral (i.e., colorless) as possible. Thus, the reflected or transmitted colors of conventional low solar heat gain coatings may be less than ideal, both in terms of hue and chroma.
Some high shading ability low-emissivity coatings that provide advantageous properties have been commercially available for years. While some of these coatings have been more than acceptable, it would be desirable that they have even better mechanical durability, chemical durability, or both.
It would be desirable to provide controlled transmission coatings based on a blocker layer material that can provide exceptional mechanical durability. It would be particularly desirable to provide such a controlled transmission coating based on a blocker layer material that also provides exceptional moisture resistance. Further, it would be desirable to provide controlled transmission coatings that also exhibit pleasing color in reflection, transmission, or both.